In the field of microfabrication represented by the manufacture of integrated circuit elements, lithographic technology enabling microfabrication with a line width of about 200 nm or less using an ArF excimer laser (wavelength: 193 nm), an F2 excimer laser (wavelength: 157 nm), and the like has been demanded in order to increase the degree of integration in recent years. As a radiation-sensitive resin composition applicable to excimer laser radiation, a number of chemically-amplified radiation-sensitive compositions utilizing a chemical amplification effect between a component having an acid-dissociable functional group and an acid generator which is a component generating an acid upon irradiation, have been proposed. For example, a high molecular-weight compound for a photoresist which comprises a resin component with a specific structure which contains a monomer unit having a norbornane ring derivative as a resin component is known (Patent Document 1 and Patent Document 2).
As a positive-tone photosensitive resin composition suitable for use with an exposure light source with a wavelength of 250 nm or less, particularly 220 nm or less, a resin in which an acid generating group, an alicyclic group, and an acid-dissociable group are introduced into the same molecule (Patent Document 3), a photosensitive resin composition containing a sulfonium or iodonium salt resin which has a counter anion in the polymer chain in order to increase photolysis efficiency (Patent Document 4), a positive-tone photosensitive resin composition having a counter anion in the polymer chain (Patent Document 5), and a negative-tone and positive-tone photosensitive resin composition of the same type (Patent Document 6) are known.
However, to achieve a higher degree of integration in the field of semiconductor devices, a radiation-sensitive resin composition used as a resist has been required to possess more excellent resolution. In addition, along with the progress of microfabrication, there is a growing demand for wider focal depth allowance (hereinafter referred to as “DOF”) and narrower line edge roughness (hereinafter referred to as “LER”) of patterns. Along with the progress of miniaturization in the semiconductor industry, development of a radiation-sensitive resin composition having excellent resolution and satisfying the demand for wide DOF and narrow LER is urgently needed.
In addition, the excellent characteristics such as sensitivity, resolution, DOF, and LER are desired to be maintained while such a radiation-sensitive resin composition is stored.
The resolution of the projection optical system provided in the projection aligner increases as the exposure wavelength used becomes shorter and the numerical aperture of the projection optical system becomes greater. Therefore, the exposure wavelength which is a wavelength of radiation used in the projection aligner has been reduced in accordance with scaling down of integrated circuits year by year, and the numerical aperture of the projection optical system has been increased.
Depth of focus is as important as resolution when carrying out the exposure. Resolution R and depth of focus 6 are respectively shown by the following formulas,R=k1·λ/NA  (i)δ=k2·λ/NA2 (ii)wherein λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. When obtaining the same resolution R, a larger depth of focus δ is obtained by using radiation with a shorter wavelength.
A photoresist film is formed on the surface of an exposure target wafer, and the pattern is transferred to the photoresist film. In a general projection aligner, the space in which the wafer is placed is filled with air or nitrogen. When the space between the wafer and the lens of the projection aligner is filled with a medium having a refractive index of n, the resolution R and the depth of focus δ are shown by the following formulas,R=k1·(λn)NA  (iii)δ=k2·nλ/NA2  (iv)
For example, when water is used as the above medium in a KrF process, the resolution R is 69.4% (R=k1·(λ/1.44)NA) and the depth of focus is 144% (δ=k2·1.44λ/NA2) in the case in which the photoresist is exposed through air or nitrogen, when the refractive index of light with a wavelength of 248 nm is n=1.44.
Such a projection exposure method to transfer more detailed patterns by reducing the wavelength of emitted light is called a liquid immersion lithographic method and is regarded as an essential technique for miniaturizing lithography, particularly lithography of the order of several tens of nanometers, and a projection aligner for the liquid immersion lithography is known (Patent Document 7).
In liquid immersion lithography, a photoresist film applied and formed on a wafer and a lens of the projection aligner respectively come into contact with an immersion medium such as water. The immersion medium may permeate into the photoresist film and reduce the photoresist resolution. Other problems are elution of a photoresist component into the immersion medium and pollution of the lens surface with such a photoresist component.
For these reasons, a photoresist film is demanded to maintain stability in the liquid immersion medium, that is, excellent liquid immersion resistance, without being eluted into an immersion medium such as water during liquid immersion lithography, and the exposed area thereof is required to be easily dissolved in an alkaline solution used as a developer.
However, no radiation-sensitive resin composition which can produce a stable film against an immersion medium such as water during liquid immersion lithography, while exhibiting excellent resolution has been obtained.    [Patent Document 1] JP-A-2002-201232    [Patent Document 2] JP-A-2002-145955    [Patent Document 3] JP-A-10-221852    [Patent Document 4] JP-A-9-325497    [Patent Document 5] JP-A-2005-84365    [Patent Document 6] U.S. Pat. No. 5,130,392    [Patent Document 7] JP-A-11-176727